Enzyme catalysts are useful for chemical transformations in industrial processes. Benefits of enzyme catalysts include catalytic activity at relatively low temperatures, unique reactions and products, and the potential for high selectivity. Immobilization on a solid support stabilizes an enzyme and prolongs its catalytic lifetime. Immobilization also facilitates removal of the catalyst from the reaction for re-use.
Enzymes are typically supported on particles, with ion exchange resins and modified silica particles among the most common supports. Enzymes immobilized on ion exchange resins are very common. Weakly acidic and weakly basic ion exchange resins are used for immobilizing enzymes. Most ion-exchange resins are based on crosslinked polystyrene in the form of small beads. In a batch reaction, agitation can pulverize the ion exchange resin beads, which reduces enzyme activity, complicates catalyst removal from the process and prevents re-use of the immobilized catalyst. Silica particles used to immobilize enzymes are often very small, complicating removal from a batch reaction. In a packed bed column, the packing of the ion exchange particles or the small particle size of the typical silica particle can result in high back pressures and difficulty flowing a reaction mix through the catalyst bed, and can eventually lead to plugging. In any reaction configuration, ion exchange particles are subject to degradation or even additional crosslinking in the reaction conditions such as in presence of solvents, at high temperatures, or in extreme pH conditions.
Exclusion membranes are small-pore membranes which restrict the movement of a homogeneous enzyme or whole-cell catalyst in a reaction system. In exclusion membranes, the enzyme is active in solution on one side of the membrane rather than immobilized on both sides of the membrane. Indeed, the membrane is used only for selective separation, and the optimal pore size is restricted by the size of the enzyme catalyst and the product. The effective pore size must exclude the enzyme catalyst while allowing the product to pass through. Biphasic and asymmetric membrane reactors employ an enzyme catalyst deposited by filtration as an insoluble particle at the interface of a hydrophobic and hydrophilic membrane to facilitate a hydrolysis reaction on the hydrophilic side of the reactor system. The hydrophilic membrane must have a pore size smaller than the size of the enzyme catalyst so as to exclude transmission of the enzyme catalyst into the aqueous phase, thus serving as an exclusion membrane. Asymmetric porous fluoropolymer membranes have been surface-modified to yield a hydrophobic and a hydrophilic surface. An enzyme catalyst is immobilized to only one surface of the asymmetrical membrane.
One object of the present invention is to overcome the limitations presented by immobilizing enzyme catalysts onto a particulate support, namely the size and structural weaknesses and single use feature. Another object of the invention is to overcome the limitations presented by association of an enzyme catalyst with an exclusion or asymmetrical porous support, namely, the use of only one surface.